1. Technical Field of the Invention
The invention relates to the field of positioning devices, i.e. of finely positionable actuators as for example used for scanning tunnelling microscopes and other scanning microscopes, so-called scanning probe microscopes. In particular, the invention relates to the construction of such a multiaxis actuator which can carry out three-dimensional movements of a sensor or a scanning tip with high regulating accuracy while being of comparatively robust and compact design. Furthermore, the invention relates to a measuring head for a scanning probe microscope comprising such an actuator. Appropriate integration of the actuator results in a robust measuring head which is simple in operation and which has advantages when compared to the state of the art.
2. State of the Art
In scanning probe microscopy, positioning of the scanning tip has been a somewhat problematic task from the very start because such positioning needs to be at the highest possible resolution while maintaining as large a regulating range as possible. In addition, good repeatability, i.e. adjustability as far as possible free of any hysteresis, is required, as is little mass, if for no other reason than that of low energy consumption. When the desire for rational, economical production of a respective measuring head is taken into account as well, it becomes evident why a host of different approaches to solving this problem of positioning scanning tips have been made. Piezo actuators are in use as are for example electromagnetic or electrostatic actuator elements for the actuators. Many of today""s scanning probe microscopes (SPMs) use piezo actuators for the necessary fine positioning which takes place in the micrometer to sub-nanometer range. But there are also other approaches as will be shown below.
However advantageous piezo actuators are in several respects, they also have a number of disadvantages, among them in particular the expensive electrical control devices and the comparatively high operating voltages, often in the 100 V or 1000 V range, with the need for respective protective devices. Furthermore, linearity is not always achieved as desired and might have to be improved by applying control technology. In addition, hysteresis can be a problem which is difficult to solve. Furthermore, extended regulating distances which after all are almost always desirable, can only be realized with relatively large arrangements. Nonetheless, due to their comparatively robust design and good value, piezo actuators have established a firm position in the market.
While other attempts at solving the problem have always been pursued, e.g. magnetostrictive, electrostrictive or electrostatic approaches, so far they do not seem to be suitable for large regulating distances, i.e. actuator regulating distances around or in excess of 50 xcexcm. This has kept a limit on possible applications and therefore also on the interest in such actuators.
By contrast, electromagnetic actuators, often referred to as voice coil arrangements, have attracted considerable interest right from the start. They are used in a host of different applications, above all outside the area of scanning probe microscopy. One such example, from the area of consumer electronics, are CD reading heads which are positioned by multidimensional actuators. Electromagnetic actuators are inherently quite linear, they can cover comparatively large regulating distances up to the mm range, and they can be operated at low voltage. Often a voice coil arrangement with stationary permanent magnet and movable coil is used but other arrangements are also known.
Such an arrangement for use in a scanning probe microscope (or a respective memory) is described by G. Binnig et al in PCT patent application WO 96/07074: xe2x80x9cFine positioning apparatus with atomic resolutionxe2x80x9d. This arrangement provides for two inductive actuator units with a positioning accuracy of better than 1 nm. A mechanical damping device acts as a brake and, together with a special mechanical arrangement, as a variable step-down device of the electromagnetic actuator. While this is intended to improve the positioning accuracy of the entire device, it renders the overall design relatively expensive and heavy. This significantly limits the range of possible applications. Nor is an arrangement in the form shown conducive to economical production. For example, the electrical cables to the movable voice coils also need to be movable which can be problematic for production as well as for reliability.
A further approach was described by S. T. Smith et al in the journal xe2x80x9cRev. Sci. Instrumentsxe2x80x9d, vol. 65, no. 4 April 1994, on pages 910 ff., entitled xe2x80x9cA simple two-axis ultraprecision actuatorxe2x80x9d. It shows an actuator comprising two actuator units, one of which is used for translatory positioning, the other for angular positioning. Reportedly, translatory distances of 80 xcexcm have been achieved, with angular ranges of 3.6 mrad being achieved at a positioning accuracy of 0.6 nrad. While this actuator only covers two axes, which would limit its application possibilities, it nevertheless provides a pointer as to what an actuator or relatively simple design might look like. But it must not be overlooked that the actuator shown by S. T. Smith, according to the presentation in the above-mentioned article really only seems suitable for illustrating the theory and general function. It is not well-developed from a production-technology point of view, indeed production-technology considerations do not seem to have featured at all in its design. At any rate it could hardly be considered a model for a robust actuator which is relatively simple to produce. Even more importantly, there is no description as to how the actuator described might be used in three dimensions.
A further, quite interesting approach is shown by R. Garcia Cantu et al in their article xe2x80x9cInductoscanner Tunneling Microscopexe2x80x9d, in Surface Science 181, pp. 216-221, Elsevier Science Publishers B.V. 1997. The actuator for a scanning tunnelling microscope described comprises a pair of diametrically opposed voice-coil actuator elements with a support, with the scanning tip extending between said actuator elements which are advantageously operated at low voltage. Suspension is via metal membranes, similar to that in an aneroid barometer. The authors do not however demonstrate resolution in the sub-nm range as required for the desired application and thus do not meet the objective set by the invention. Moreover, the selected arrangement of the opposing voice coil actuators appears to be somewhat voluminous and hardly suitable for economical production. In addition, large volume usually results in low self resonance which is not always desirable, and substantial mass and the associated large working surface almost always means a high susceptibility to external interference.
As can be seen from the above, it is the object of the invention to create a three-dimensionally controllable electromagnetic actuator, in particular for scanning probe microscopes, with said actuator being of comparatively simple and robust construction with regulating distances as large as possiblexe2x80x94preferably in the 100 xcexcm rangexe2x80x94while at the same time providing good positioning accuracy xe2x80x94preferably in the sub-nm rangexe2x80x94and high linearity as far as possible without external feedback, especially without mechanical feedback. In addition, the arrangement should be compact and as simple as possible to produce.
This rather complex object is met in principle relatively simply by the invention by the arrangement of preferably three electromagnetic actuator units arranged in one plane and/or being essentially identical, said actuator units being connected to a rigid fulcrum structure supporting the scanning tip such that said fulcrum structure, depending on the activation of the actuator units, can be tilted on any desired rotary axis in said plane and/or can be moved in a translatory way essentially perpendicularly to said plane, as a result of which the scanning tip, which is situated outside said plane, can be moved in space quasi in any way desired.
It would be advantageous, but this is not a condition, if the three actuator units were to act in the same direction of action and in addition, if they were positioned in a suitable plane. In this way, the spring provided for each actuator unit could be advantageously configured as a part of a diaphragm spring shared by all actuator units, thus reducing construction expenditure. Below, further advantageous embodiments are shown in the description of an embodiment and in the claims.